2, JST, Kawaguchi, Saitama, Japan
Organic semiconductors (OSCs) attracts great interest for the next generation electronics and spintronics applications. Recently, a new class of single-crystal OSCs with a mobility exceeding 10 cm2V-1s-1 has been developed, in which a band-like charge transport is realized thanks to its highly periodic electrostatic potential despite the weak van der Waals coupling. Additionally, OCSs are the candidate materials for the spin media due to their long spin relaxation time, typically 10-7—10-6 s, which is several orders longer than that of inorganic materials due to weak spin-orbit interaction. However, spin transport and relaxation mechanism of the single-crystal OCSs are not extensively investigated because of the technical difficulties of the spin detection.
Here, in our report , we applied operando-ESR technique to high-mobility single-crystal OCSs, and addressed the charge and spin relaxation mechanisms. Operando-ESR of single-crystal organic field effect transistor (OFET) allows to measure intrinsic spin dynamics of the gate-induced carriers. To address intrinsic charge momentum and spin relaxation mechanisms, a single crystal of our benchmarked OSC, C10–DNBDT–NW , was grown directly on a substrate via the continuous edge-casting method that has been developed by our group. This method produces an ideal single crystal with over an inch in size, which allows to observe ESR signal with the good signal-to-noise ratio. For the precise evaluation of momentum relaxation time τp, firstly, the band-like transport is evidenced by the observation of Hall effect and the negative temperature (T) dependence of mobility. The measured mobility, and thus τp, clearly increases as T decreases, where the temperature dependence of mobility can be fitted with T-0.85. Spin-lattice relaxation time T1 and spin-spin relaxation time T2 were estimated by operando-ESR. T1, T2, and τpT-2 respect to temperature agree perfectly with T-2.85; that is, they satisfy the relationships predicted from Elliott-Yafet mechanism. This leads to the conclusion that the EY mechanism is dominant in OSCs. The EY mechanism describes the spin conserving/flipping probability at an ordinary scattering event with the presence of SOC.
The demonstrated operando-ESR measurement would be useful to fully understand the charge and spin transport in OSCs under low-temperature regimes. On the basis of the verified EY mechanism, T1 ∝τpT-2 holds even at low-temperature regimes, and the intrinsic drift mobility free from any traps and defects may reach 650 cm2V-1s-1 at 4 K. In addition, the spin diffusion length is estimated to be 840 nm at 300 K, and is likely to approach 1.6 μm at 50 K. The extraordinarily long spin diffusion length is expected due to the coexistence of ultra-long spin coherence times and relatively high band-like mobility.
 J. Tsurumi et al., Nature Phys. 13, 994 (2017).  C. Mitsui et al. Adv. Mater. 26, 4546 (2014).